Automatic variable attenuator circuit



Nov. 16, 1965 v. GODIER AUTOMATIC VARIABLE ATTENUATOR CIRCUIT Filed Jan.28, 1963 A.L.C. UNIT INVENTOR IVAN GODIER BY M w TTORNEYS.

United States Patent ()fifice 3,218,570 Patented Nov. 16, 1965 3,218,570AUTOMATIC VARIABLE ATTENUATOR CIRCUIT Ivan Godier, Ottawa, Ontario,Canada, asslgnor to Northern Electric Company Limited, Montreal, Quebec,Canada Filed Jan. 28, 1963, Ser. No. 254,072 3 Claims. (Cl. 330-143)This invention relates to automatic level control systems and morespecifically provides an improved variable attenuator for such a system.

Automatic level control systems employing thermistors as controlelements are well known for reducing the undesirable variations in theoutput signal level of transmission systems. Such variations may becaused by changes in attenuation of the interconnecting cablesassociated with the system, with ambient temperature and/ or variationsin gain of the associated amplifiers.

These control systems are particularly useful in broadband amplifierssuch as those used in video distribution systems with frequenciesextending into the VHF region. When extremely low distortion is aprerequisite in a video distribution system, it is generally notpossible to directly vary the gain of the active elements in theassociated amplifiers. Any such attempt would derange the operatingconditions of the amplifier and increase the distortion above atolerable level.

In one such control system, a single thermistor is placed either inseries or in shunt with the input to the system and the impedance of thethermistor is altered, in accordance with variations in the outputsignal level, by a control network in such a direction so as to reducethe undesirable output signal level variations. If, as stated above,only one thermistor is located at the system input, variations in thethermistor impedance also affect the system input impedance, and may,therefore, cause undesirable reflections if connected to the end of along line transmission system. This can be overcome at additional costby utilizing a buffer amplifier between the system input and theattenuator input. Besides the additional cost, the use of a bufferamplifier may unnecessarly introduce additional distortion into thesystem.

An alternate method of maintaining the system input impedance constantis to use a bridge-T network containing two thermistors. One of the twothermistors is used to control the attenuation of the network inresponse to variations in the output signal level, while the otherthermistor is used to control the impedance of the network in responseto an unbalance created in the normally balanced series arm of thebridge-T network. One disadvantage of this system is that a separatecontrol circuit must be used for each thermistor; one across the systemoutput and the other across the normally balanced series arm of thebridge-T network. Another disadvantage is the series impedance of thebridge-T network across which the unbalance is detected is ungrounded.In the VHF band, undesirable mismatches may be introduced by such aconfiguration,

The disadvantages of the present systems may be overcome according tothe present invention by utilizing two thermistors to control both theattenuation and input impedance of the system through a single unitcontrolled by variations in the output signal level. Thus a sample ofthe output level is fed to an automatic level control unit whichproduces a D.C. error signal proportional to the output level. The errorsignal controls the power to a separate heater associated with eachcontrol thermistor in such a manner as to differentially vary the powerto the two thermistor heaters. The impedance of the thermistors willthen vary inversely with respect to each other.

If reciprocal relationship is maintained between the impedance of thetwo thermistors, the input impedance and,

if desired, the output impedance of the attenuator network can bemaintained constant.

In the drawings which illustrate embodiments of the invention,

FIGURE 1 is a schematic circuit diagram of an automatic level controlsystem having one form of a variable attenuator network according to theinvention, and

FIGURES 2 and 3 show modifications of the variable attenuator network ofFIGURE 1.

In the circuit of FIGURE 1, the system input is connected directly toattenuator 10, the output of which is connected to amplifier 11. Theoutput of the amplifier is connected to the output of the system. Asample voltage from the output is fed to the automatic level controlunit 12. This unit develops a varying D.C. error voltage proportional tothe output signal level and may be of conventional design. The errorvoltage is fed through connecting lead 13 to control transistor 14.

The attenuator 10 comprises a first thermistor 15 serially connected toa first predetermined fixed impedance 16 and in shunt across the inputof the attenuator 10. A second thermistor 17 is serially connectedbetween the input and the output of the attenuator 10. A first controlheater 18 is associated with thermistor 15 and a second control heater19 is associated with the second thermistor 17. The input impedance tothe attenuator will be maintained constant and equal to the lineimpedance providing:

(1) The impedance of fixed impedance 16 is made equal to the lineimpedance Z (2) The impedance of the two thermistors 15 and 17 arereciprocally controlled and equal to the line impedance Z when they areequal to each other.

Thus:

Z is the line impedance,

Z is the impedance of the first thermistor,

Z is the impedance of the second thermistor, Z is the impedance of thefixed impedance 16.

The power to the heaters of the thermistors 15 and 17 is suppliedthrough a high resistance 23 from a D.C. source of power 24 providing aconstant current source 25. The first control heater 18 and a fixedresistor 26 are serially connected across the constant current source25. The second control heater 19 is serially connected to the controltransistor 14, the combination being in shunt with the first controlheater 18 and the fixed resistor 26, across the constant current source25. Because two control heaters 18 and 19 are fed from a constantcurrent source 25, the sum of the currents through the control heaterswill always equal a constant.

For example:

I is the current through the first control heater. I is the currentthrough the second control heater. K is a constant.

Thus, if an error signal on connecting lead 13 shuts control transistor14 01f, all the current from the constant current source 25 will flowthrough the control heater 18. Conversely, if transistor 14 isconducting heavily the impedance of the transistor 14 is then relativelylow compared to the fixed resistor 26, and virtually all the currentfrom the constant current source 25 will fiow through the control heater19. Thus, the total current from the constant current source 25, splitsin a differential manner between the two heater 18 and 19, in accordancewith the amplitude of the error signal on the lead 13. Because theimpedance of thermistors 15 and 17 vary in accordance with theirtemperatures, they will be forced to vary inversely with respect to eachother, when the power to the control heaters 18 and 19 is varieddifierentially.

An alternate form of input attenuator shown in FIGURE 2 comprises firstthermistor in shunt across the output of the attenuator, and secondthermistor 17 in shunt with a second fixed impedance serially connectedbetween the attenuator input and output. The control heaters 18 and 19associated with thermistors 15 and 17 respectively, are also shown, Thethermistor requirements for a constant input impedance to the attenuatorinput are similar to those indicated above. The second fixed impedance20 must also be made equal to the line impedance Z A third form of inputattenuator 16 shown in FIGURE 3 comprises a bridge-T network containinga third and fourth fixed impedance series arms 21 and 22 respectively,located between the input and the output of the attenuator 10, a firstthermistor 15 shunt arm, and a second thermistor 17 bridging arm.Control heaters 18 and 19 are again associated with thermistors 15 and17 respectively. The advantage of the bridge-T configuration over theother two is that both the input and the output impedances of theattenuator are maintained constant providing the two fixed impedances 21and 22 are made equal to the line impedance Z the two thermistors 15 and17 are varied reciprocally and are equal to each other when they areequal to the line impedance Z What I claim as my invention is:

1. An automatic variable attenuator circuit comprising input connectionsfor connecting an input signal voltage thereto, output connections forconnecting an output signal voltage therefrom; an amplifier having aninput and an output, the amplifier output being connected to said outputconnections; a variable attenuator network connected between the inputconnections and the amplifier input, and including a first thermistorand a fixed impedance connected in series across said input connectionsand a second thermistor connected in series between the inputconnections and the amplifier input; a first heater in juxtapositionwith the first thermistor; a second heater in juxtaposition with thesecond thermistor; means for generating a variable direct current errorsignal that is proportional to the output signal voltage level; a highimpedance source of direct current power; a transistor having base,emitter and collector electrodes, the emitter and collector electrodesbeing connected in series with the second heater across the highimpedance source, the base and emitter electrodes being connected to themeans for generating a variable direct current error signal; and a fixedresistor connected in series with the first heater across the highimpedance source.

2. An automatic variable attenuator circuit comprising input connectionsfor connecting an input signal voltage thereto, output connections forconnecting an output signal voltage therefrom; an amplifier having aninput and an output, the amplifier output being connected to said outputconnections; a variable attenuator network having a first thermistorconnected in shunt with the amplifier input, and a second thermistor anda fixed impedance connected in shunt with each other between the inputconnections and the amplifier input; a first heater in juxtapositionwith the first thermistor; a second heated in juxtaposition with thesecond thermistor; means for generating a variable direct current errorsignal that is proportional to the output signal voltage level; a highimpedance source of direct current power, a transistor having base,emitter and collector electrodes, the emitter and collector electrodesbeing connected in series with the second heater across the highimpedance source, the base and emitter electrodes being connected to themeans for generating a variable direct current error signal; and a fixedresistor connected in series with the first heater across the highimpedance source.

3. An automatic variable attenuator circuit comprising input connectionsfor connecting an input signal voltage thereto, output connections forconnecting an output signal voltage therefrom; an amplifier having aninput and an output, the amplifier output being connected to said outputconnections; a bridge-T network connected between the input connectionsand the amplifier input, and including two fixed impedance series arms,a shunt arm having a first thermistor, and a bridging arm having asecond thermistor; a first heater in juxtaposition with the firstthermistor; a second heater in juxtaposition with the second thermistor;means for generating a variable direct current error signal that isproportional to the output signal voltage level; a high impedance sourceof direct current power; a transistor having base, emitter and collectorelectrodes, the emitter and collector electrodes being connected inseries with the second heater across the high impedance source, the baseand emitter electrodes being connected to the means for generating avariable direct current error signal, and a fixed resistor connected inseries with the first heater across the high impedance source.

References Cited by the Examiner UNITED STATES PATENTS 2,182,329 12/1939Wheeler 330144 X 2,250,581 7/1941 Heinecke 330-143 X FOREIGN PATENTS131,275 2/1949 Australia. 664,644 1/ 1952 Great Britain.

OTHER REFERENCES Ryder: Networks, Lines and Fields, Prentice-Hall,Englewood Cliffs, N.J., 1955, 2nd edition, pages 267-8 relied on.

ROY LAKE, Primary Examiner.

1. AN AUTOMATIC VARIABLE ATTENUATOR CIRCUIT COMPRISING INPUT CONNECTIONSFOR CONNECTING AN INPUT SIGNAL VOLTAGE THERETO, OUTPUT CONNECTIONS FORCONNECTING AN OUTPOUT SIGNAL VOLTAGE THEREFROM; AN AMPLIFIER HAVING ANINPUT AND AN OUTPUT, THE AMPLIFIER OUTPUT BEING CONNECTED TO SAID OUTPUTCONNECTIONS; A VARIABLE ATTENUATOR NETWORK CONNECTED BETWEEN THE INPUTCONNECTIONS AND THE AMPLIFIER INPUT, AND INCLUDING A FIRST THERMISTORAND A FIXED IMPEDANCE CONNECTED IN SERIES ACROSS SAID INPUT CONNECTIONSAND A SECOND THERMISTOR CONNECTED IN SERIES BETWEEN THE INPUTCONNECTIONS AND THE AMPLIFIER INPUT; A SECOND HEATER IN JUXTAPOSITIONWITH THE FIRST THERMISTOR; A SECOND HEATER IN JUXTAPOSITION WITH THESECOND THERMISTOR; MEANS FOR GENERATING A VARIABLE DIRECT CURRENT ERRORSIGNAL THAT IS PROPORTIONAL TO THE OUTPUT SIGNAL VOLTAGE LEVEL; A HIGHIMPEDANCE SOURCE OF DIRECT CURRENT POWER; A TRANSISTOR HAVING BASE,EMITTER AND COLLECTOR ELECTRODES, THE EMITTER AND COLLECTOR ELECTRODESBEING CONNECTED IN SERIES WITH THE SECOND HEATER ACROSS THE HIGHIMPEDANCE SOURCE, THE BASE AND EMITTER ELECTRODES BEING CONNECTED TO THEMEAANS FOR GENERATING A VARIABLE DIRECT CURRENT ERROR SIGNAL; AND AFIXED RESISTOR CONNECTED IN SERIES WITH THE FIRST HEATER ACROSS THE HIGHIMPEDANCE SOURCE.